innermost planet - .4 AU

Mass - only 5% of Earth's

highly ellipical orbit

40% vs 4% Earth


long believed to be in synchronous rotation - Doppler radar has changed that belief

59 day sidereal rotation
it has an 88 day revolution so each sidereal day is 2/3 of the year
188 day solar rotation
so each solar day is two Mercury years long

It is a morning or evening "star" never more than 28° away from the sun ~two hours before or after sunrise

Albedo - fraction of radiation reflected

6% which is low (100% is everything) and about the same as the Moon (Why is the moon so bright?)


We know its angular diameter, its period - we can calculate its mass. Then we can determine its denisty

Topographic features


Caloris Basin